Optical sensors have clear advantages namely sensitivity; accuracy; wide dynamic range; and the inherent immunity to thermal and electromagnetic disturbances. Fiber-optic sensors offer the advantage of compactness.
Fiber-optic sensors may be classified in two major categories: intrinsic, where the measured parameter interacts with the guided light inside the fiber; and extrinsic where the measured parameter interacts with the light outside the fiber. In extrinsic sensors the fiber or waveguide is used only to guide light to the location of the interaction.
In intrinsic sensors long loops of fiber (up to a few hundred meters for hydrophones) are needed to accumulate appreciable signal because sensitivity of optical characteristics of the fiber to a measured parameter is small. However, such long fiber cables become more sensitive to parameters other than the one being sensed. Fiber optic hydrophones which are used for sensing sound transmitted through water, for example, are more susceptible to random fluctuations in temperature along the fiber than to pressure variations. Further, the optical fiber supports two orthogonal modes of polarization. This creates mode hopping, i.e. the state of polarization of the output fluctuates in a long fiber and introduces extraneous noise. Also performance of intrinsic sensors is critically dependent on opto-mechanical properties of the fiber which may vary from batch to batch or over time, thus requiring frequent calibration.
Extrinsic sensors, on the other hand, do not rely on the length or properties of the fiber to amplify the signal and are therefore more compact, reliable, and less susceptible to outside disturbance. Further, volume between fibers in an extrinsic sensor can be reduced tremendously to produce true point sensors, desirable in situations where the field to be measured varies appreciably along the length of a transducer.
Fiber optic sensors can be further classified as interference or intensity types depending on the property of light modulated by interaction with the measured parameter. Interference is a phenomenon where the superposition of two or more waves creates a change or redistribution in the intensity of the light which is related to the relative phase difference between the waves. The phase change is caused by the parameter which we desire to measure. Thus the parameter can be inferred from the phase measurement. Phase modulation by interference is more accurate and sensitive than intensity modulation because the path length change is referenced to the wavelength of light. Interference sensors have better performance and wider dynamic range than intensity sensors because of the elaborate feedback schemes commonly used in conjunction with phase modulation.
Interferometric sensors employ single mode fibers or waveguides where the core diameter is of the order of a few wavelengths (5-10 microns), while intensity type sensors use multimode fibers whose core is up to 100 microns in diameter. In extrinsic sensors light must reenter a fiber after interaction with the parameter to be measured. This is very difficult to do with a single mode fiber because of tight alignment tolerances that must be observed. This explains in part why most optical fiber sensor development in the last one and a half decades since its inception have been either intrinsic interferometric or extrinsic intensity type sensors. Two authors who have written extensive surveys on fiber optics sensors: D. H. McMahon in "Fiber-Optic Transducers," IEEE Spectrum, December 1981, pages 24 and 26; and W. B. Spillman in "Multimode fiber-optic hydrophone based on a schlieren technique," Applied Optics, February 1981, page 465, assert that single mode (interference) sensors are mostly intrinsic whereas multimode (intensity) type sensors are usually extrinsic. No mention is made of extrinsic interferometric sensors. U.S. Pat. No. 4,421,384 discloses an extrinsic fiber-optic acoustic sensor where coupling mechanism across air gap between two multimode fibers depends on frustrated total internal reflection which is an intensity rather than interference effect. Also U.S. Pat. No. 4,293,188 discloses an extrinsic fiber-optic displacement sensor with a gap between two multimode fibers effecting intensity modulation. It uses cylindrical GRIN lenses at fiber ends centered on fiber axis. A variation uses diffraction gratings on facets of fiber ends to shadow parts of light beam and produce intensity modulation. Lack of progress based on extrinsic interferometric principles is in part due to lack of transduction techniques to convert small displacements into proportional phase changes.
In an extrinsic sensor light is ejected from the end of a fiber or waveguide and reflected off an external mirror either into the same or another fiber. Between the two fiber ends light is unguided, and it may or may not be collimated. A displacement sensor measures motion of external mirror relative to the rest of the interferometer as affected by environmental excitations such as acceleration, displacement or acoustic signal. Motion of the mirror alters the path length of the light and produces phase modulation in the interferometer. If the sensor measures displacement or proximity of a nearby object then the mirror is detached from the rest of the interferometer and affixed to the moving body. For example the Atomic Force Microscope (AFM) has a tiny mirror attached to the tip of a miniature cantilever beam which is made to track the surface topography of a sample down to subatomic resolution. If a sensor measures acceleration or vibration, the mirror is mounted on a flexible element such as a thin diaphragm or a cantilever beam and the whole interferometer is attached to the vibrating body. Mirror displacement can also result from subjecting the interferometer to an acoustic signal as in hydrophones. D. A. Jackson, et al. describes a "high sensitivity fiber-optic accelerometer" having a spherical mirror mounted on a thin flexible diaphragm in Optics Letters, Feb. 15, 1989, page 251.
Extrinsic interferometric displacement sensors currently available require an external mirror which is mounted on a cantilever beam or a diaphragm. This adds bulk and expense to system because the cantilever must be fabricated separately, mirrored and then attached to the interferometer. Miniaturization is desired in order to take full advantage of batch fabrication techniques which are well established in microelectronic and integrated-optic industries, where a whole sensor or interferometer is fabricated in a few steps on a semiconductor substrate. Thus, configurations currently used requiring addition of an external cantilever beam do not lend themselves to miniaturization.
In the scanning AFM a sharp stylus on the tip of a microfabricated cantilever is made to track surface topography of a sample as the sample is scanned in its own plane. Interferometry has been used for the measurement of cantilever deflection. A single mode optical fiber is brought within a few microns of the cantilever tip as described by D. Rugar et al in "Improved fiber-optic interferometer for atomic force microscopy," Applied Physics Letters, Dec. 18, 1989, page 2588. This is necessary to reduce acoustic noise generated in the air gap and the effect of light source frequency fluctuations since a resolution of 0.1 Angstrom is required. This configuration which utilizes an external mirror in the design of the AFM presents a few problems. Precise positioning of the fiber over the cantilever is a major alignment problem in any extrinsic interferometer. Another problem which arises in conjunction with the AFM is scanning large samples. Since the scanning range of a sample is limited it is often desirable to scan the cantilever along with its optical interrogation system, that is the interferometer instead of the sample. This is a particular problem with bulky optical systems which utilize heavy focusing lenses and laser sources, but also with fiber-optic systems in which the fiber is directed normal to the plane of the cantilever.
One solution to these problems is to lay the fiber or waveguide on top of the cantilever. Fiber cantilevers have been proposed for the extrinsic measurement of displacement. Sometimes the fiber was strapped onto a thin cantilever or a thin film waveguide was deposited on a microfabricated cantilever. However, all these sensors have been of the intensity modulation type because the fiber or waveguide is aligned with the neutral axis of the cantilever. This is certainly true when the fiber itself is used as the cantilever. In the other instances a microfabricated cantilever is thin enough so that the fiber is substantially aligned with the neutral axis. It is known that a neutral axis moves only transversely to itself under cantilever flexure. It has remained a problem, however, to transduce a path length or phase change proportional to this lateral displacement. This explains why most extrinsic sensors have been of the intensity type. Proportionality is important to simplify signal processing and to obtain a sizable signal in the case of small displacements.